1. (Field of the Invention)
The present invention relates to a multilayer wiring board and, particularly, to a multilayer wiring board adapted to packages for containing semiconductor elements, and to a process for producing the same.
2. (Description of the Prior Art)
A ceramic multilayer wiring board capable of forming wirings at a relatively high density has heretofore been used for packaging semiconductor elements. The ceramic multilayer wiring board is formed of an insulating board such as alumina and a wiring conductor composed of a high-melting metal such as tungsten or molybdenum formed on the surface thereof. Cavities are formed in portions of the insulating board, and semiconductor elements are held in the cavities which are hermetically sealed by a closure member.
Such a ceramic multilayer wiring board is hard and brittle, and is subject to be broken or cracked during the step of production or the step of conveyance, and loses hermetic sealing for the semiconductor elements, resulting in a decrease in the yields of the final products. The board is produced by printing a metallizing ink onto a green sheet, and laminating many printed sheets followed by sintering accompanied, however, by a problem of shrinking due to firing at a high temperature in the step of sintering. In other words, the obtained board is deformed such as being warped and undergoes dimensional variation, and is not capable of sufficiently meeting strict requirements such as very high density of wiring, flatness required for the boards, such as flip chip, etc.
Recently, therefore, there has been proposed a multilayer wiring board of a resin obtained by adhering a copper foil onto the surface of an insulating board made of an organic resin, forming a fine wiring circuit layer on the insulating board by etching the copper foil, and laminating such an insulating board having such a wiring circuit layer one upon the other. Such a resin board is free from such defects as breakage or cracking inherent in the ceramic board. Even when laminated, furthermore, the above resin board does not require heat treatment at a high temperature like that of firing. To increase the strength of the resin board, furthermore, there has been proposed an insulating board obtained by dispersing a spherical or fibrous inorganic filler in an organic resin. Study has further been conducted to adapt the insulating substrate to a multi-chip module (MCM) which mounts many semiconductor elements.
With the above-mentioned resin board in which protruded portions are formed by a wiring circuit layer on the insulating board, however, flatness is low and fails to satisfy the requirements for mounting the flip chips. Moreover, use of the etching solution and plating solution for forming the wiring circuit layer deteriorate the characteristics of the insulating board.
Furthermore, the conventional multilayer wiring board made of a resin must be improved from the standpoint of accomplishing fineness and high density. That is, it is widely accepted practice to form through-holes penetrating through the board and to connect the wiring circuit layers by plating the interior of the through-holes. Here, however, the through-holes impose limitation on the circuit design making it difficult to accomplish high-density wiring. There has further been developed a so-called build-up method according to which an insulating layer is formed by applying an insulating slurry and a wiring circuit layer is formed by plating alternatingly on the surface of the board and, at the same time, via holes are formed in the insulating layers to obtain a laminate of many layers. Even with the build-up method, however, limitation is imposed on arranging the via holes, making it difficult to accomplish high-density wiring. Furthermore, the number of times the insulating layers are immersed in the etching solution and in the plating solution increases with an increase in the number of the layers laminated on the board to accomplish the wiring circuit of a high density. Therefore, a problem remains in that the insulating layer absorbs these chemical solutions and is degraded. Thus, it is difficult to accomplish the high-density wiring even relying upon the build-up method.
In recent years, therefore, it has been proposed to form via hole conducting passages by filling the via holes with a conducting paste. This method makes it possible to form via hole conducting passages at any place and is drawing attention as technology which is indispensable for forming high-density wiring.
When the via hole conducting passages are formed by filling the via holes with the conducting paste, however, there develops a defect in that the circuit becomes defective. That is, the conducting paste is obtained by dispersing a metal powder in the resin binder. Therefore, the via hole conducting passages formed as described above contain voids in large amounts between the metal particles. When a wiring circuit layer that is electrically connected to the via hole conducting passages is formed by intimately adhering a metal foil to the via hole conducting passages formed in the insulating layer, by applying a resist, by effecting the etching, and by peeling off the resist, however, the etching solution and the resist-removing solution enter into the voids in the via hole conducting passages and cause the circuit to become defective.